1. Field of the Invention
The present invention relates to a vector control method for a spindle motor, and more particularly, to a control method for a spindle motor capable of high-accuracy contour machining.
2. Description of the Related Art
It is known to perform vector control for speed control of a spindle motor mounted in a machine tool, which motor is comprised of a thee-phase induction motor, for instance. As shown in FIG. 2, a conventional vector control apparatus comprises a speed controller 1 operable to perform proportional-plus-integral control in accordance with a deviation between a speed command .omega.c and an actual speed .omega.or of a spindle motor (not shown) detected by a speed sensor (not shown) to thereby obtain a torque command Tc, and magnetic flux command means 2 for obtaining a magnetic flux command .PHI.c in accordance with the torque command Tc and the motor speed .omega.r.
A curve (FIG. 3) which indicates the maximum value .PHI.cmax of the magnetic flux command, represented as a function of the motor speed .omega.r, and the minimum value .PHI.cmin of the magnetic flux command are set beforehand in the magnetic flux command means 2. The maximum magnetic flux command curve is set so as to take a fixed value .PHI.max in a region where the motor speed .omega.r is not higher than a magnetic flux attenuation starting speed .omega.p which corresponds to the DC link voltage of an inverter (not shown), and to give, in a region where the motor speed exceeds the value .omega.p, a maximum magnetic flux command .PHI.cmax which is decreased as the motor speed increases. The magnetic flux command means 2 calculates the torque command Tc in accordance with equation (1), by using the maximum magnetic flux command .PHI.cmax obtained from the maximum magnetic flux command curve in accordance wit the motor speed .omega.r, the torque command Tc, and the minimum magnetic flux .PHI.cmin. As shown in FIG. 4, the magnetic flux command .PHI.c changes in proportion to the square root of the torque command Tc, the command being equal to the maximum magnetic flux command .PHI.cmax determined in dependence on the motor speed .omega.r when the torque command takes the maximum value Tcmax, and being equal to the minimum value .PHI.cmin when the torque command is zero. EQU .PHI.c=Tc/Tcmar .sup.1/2 .multidot.(.PHI.cmax-.PHI.cmin)+.PHI.cmin.(1)
Referring again to FIG. 2, secondary current command means 3 divides the torque command Tc supplied from the speed controller 1 by the magnetic flux command .PHI.c supplied from the magnetic flux command means 2, thereby determining a secondary current command I2C. Magnetic flux current means 4 divides the magnetic flux command .PHI.c by a constant k1, to determine a magnetic flux current command IO. In slip speed calculating means 5, the product of the secondary current command I2C and the rotor winding resistance R2 of the spindle motor is divided by the product of the magnetic flux command .PHI.c and a constant k2, thereby obtaining a slip speed .omega.s. Driving frequency command means 6 adds the moor speed .omega.r to the slip frequency .omega.s, thereby obtaining a driving frequency .omega.O of the spindle motor. The vector control based on the secondary current command I2C, the magnetic flux current command IO, and the during frequency .omega.O delivered from the elements 3, 4 and 6, respectively, is executed by means of vector control means 7, whereby a primary current command IC is generated. Induced voltage estimating means 8 for respective phases (only one for one phase is shown in FIG. 2) calculate estimated induced voltages EO or the individual phases with a phase difference of 2.pi./3 from one another, in accordance with a constant k3 and the magnetic flux command .PHI.c and the driving frequency .omega.O delivered from the elements 2 and 6, respectively. In current controllers 9 for the individual phases (only one for one phase is shown), a current feedback value I.sub.f for each phase, detected by means of a current sensor (not shown), is subtracted for the primary current command Ic supplied from the element 7, whereby a voltage command Vc for each phase is generated. Then, pulse width modulation (PWM) processing is executed in accordance with a compensated voltage command Vc, obtained by adding the estimated induced voltage EO supplied from the element 8 to the voltage command Vc supplied form the element 9, and the spindle motor is driven through the medium of the inverter.
In the machine tool equipped with the vector control apparatus of FIG. 2, the spindle motor is driven in any desired mode including a low-speed operation mode, a high-speed operation mode, an orientation mode where the spindle is positioned for automatic tool replacement, and a control control mode (Cs contour control mode) for control machining of a workpiece mounted on the spindle. Conventionally, the torque command .PHI.c is calculated in accordance with equation (1), irrespective of the drive mode of the spindle motor. Meanwhile, a magnetic flux actually produced in the spindle motor is subject to a delay behind the magnetic flux command .PHI.c due to the presence of inductance. As a result, them toro speed is subject to irregularity, so that vibration occurs in the motor. In the high-speed operation mode or the orientation mode, the irregularity of the motor speed causes no substantial hindrance. In the low-speed operation mode, however, the irregularity of the motor speed attributable to the delay of the actual magnetic flux makes it difficult to effect positioning control with required accuracy. Also in the Cs control control mode, which requires accuracy about 100 times as high as the positioning accuracy for the orientation mode, control machining with the required accuracy sometimes cannot be achieved due to the delay of the actual magnetic flux.